Foamable polyvinyl chloride compositions containing little or no plasticizer are known. See, for example, published British patent Specification No. 1,321,706, which is hereby incorporated by reference. These compositions, with at most only minor amounts of a plasticizer component, are particularly suitable for the manufacture of extruded rigid foam articles such as tubes and other shaped members. As those in the art are aware, these foamable polymer compositions contain polyvinylchloride, or a vinylchloride copolymer, a chemical or physical blowing agent, and one or more additives, such as stabilizers, auxiliary stabilizers, substances to improve impact strength, lubricants, etc.
These foamable polyvinyl chloride compositions are extruded into shaped articles such as tubes, etc. The rigid foam tubes, and other rigid foam shaped articles, have many uses.
An important physical characteristic of these rigid foam articles is their density. It is often desirable to obtain rigid foam articles with as low a density as possible. One way of producing a low density polyvinyl chloride foam is to add large amounts of a chemical or physical blowing agent to the foamable polyvinyl chloride composition. However, such compositions are difficult to use in the production of foamed extruded articles such as tubes and shaped members. The amount of foaming agent may be so large that the foaming proceeds too vigorously, so that an irregular foam with uneven cells and an uneven surface is formed. In addition, the density of the foamed product is difficult to control and may be higher than is desired. Finally, low density extruded hard foamed articles made by this method exhibit poor dimensional stability at elevated temperatures so that such extruded articles are not suitable for applications requiring constant dimensions.
Another method of producing a polyvinyl chloride foam is by adding certain modifiers, e.g., copolymers of butadiene, styrene, and/or .alpha.-methylstyrene, and also acrylonitrile, polyacrylates, or polymethacrylates, to promote the foaming into a foam having regular cells and an even surface. However, relatively large quantities of modifier must be added, and this, is expensive, particularly as the above modifiers as such are expensive and of considerably higher price than the vinylchloride polymers.